This quest will bug out on you sometimes.
After you click on the mound, it spawns about 4-5 spiders. I imagine most people just run out of the cave and leave the spiders. This will cause it to bug out and you won't be able to click the mound. If it's bugged for you, be sure to kill ALL spiders in the area near the mound. The spiders can even bug out and not let you attack them. If that happens just keep trying to attack them and move all around the back of the cave. It took me about 5 times walking over the bugged out spider, try hitting it, then running back to the left corner of the cave. Then the spider came after me and I could kill it. Quest worked perfectly after that.